Underwater excavation apparatus

ABSTRACT

An improved underwater excavation apparatus achieves efficiency and control of movement through provision of a hollow body having at least one inlet and at least one outlet, at least one pair of impellers coaxially displaced one from the other and rotatably mounted in the hollow body, and a mechanism for driving the impellers in contrary rotating directions. The underwater excavation apparatus comprises a pair of horizontally opposed inlets communicating with a single outlet, the outlet being disposed vertically downwards substantially midway between the two inlets, in use. The excavation apparatus may, therefore, be substantially “T” or “Y” shaped. The mechanism for driving the impellers may include at least one drilling motor.

BACKGROUND OF THE INVENTION

This invention relates to an improved excavation apparatus, and inparticular to an improved underwater excavation apparatus.

Underwater excavation apparatus are known, eg, from GB 2 240 568(CONSORTIUM RESOURCE et al). In that disclosure there is described. anunderwater excavation apparatus comprising a hollow body with an inletto receive water and an outlet for discharge of water. A propeller isrotatably mounted in the hollow body to draw water through the inlet anddeliver a flow 6f water through the outlet. Water jets on the propellertips rotate the propeller when water is supplied to the jets.

Such rotation causes water to be drawn into the body through the inletand expelled from the body as a flow through the outlet. The flow can beused to displace material on the seabed.

Known prior art underwater excavation apparatus suffer from a number ofproblems/disadvantages, for example:

(a) Low energy efficiency due to e.g. hydrodynamic limitations of fluidjets, thus requiring extremely large and power hungry pumps to drive thesystem);

(b) tendency of apparatus to rotate in reaction to rotation of thepropeller;

(c) difficulty in steering and positioning of the apparatus.

SUMMARY OF THE INVENTION

It is an object of at least some of the aspects of the present inventionto seek to obviate or mitigate one or more of the aforementionedproblems in the prior art.

According to a first aspect of the present invention there is providedan underwater excavation apparatus comprising a hollow body having atleast one inlet and at least one outlet, at least one pair of impellersrotatably mounted in the hollow body and means for driving theimpellers.

Advantageously, the driving means cause the impellers to be driven incontrary rotating directions, in use.

The at least one inlet may be inclined at an angle to an axis alongwhich the at least one outlet is provided.

Preferably, there is provided at least one pair of inlets.

Preferably, the at least one pair of inlets are substantiallysymmetrically disposed around an axis extending from the outlet.

In one embodiment the underwater excavation apparatus comprises a pairof horizontally opposed inlets communicating with a single outlet, theoutlet being disposed vertically downwards substantially midway betweenthe two inlets, in use. In this case, the excavation apparatus is,therefore, substantially “T” shaped in profile.

In an alternative embodiment the underwater excavation apparatuscomprises a pair of inlets communicating with a single outlet, theinlets being substantially symmetrically disposed around an axisextending from the outlet, the outlet being disposed verticallydownwards substantially midway between the two inlets, in use. In thiscase, the excavation apparatus is, therefore, substantially “Y” shapedin profile.

Advantageously, the outlets are each spaced/inclined substantially 45°from the axis extending from the outlet.

At least one impeller may be provided within/adjacent each inlet.

The means for driving the/each impeller(s) may include at least onedrilling motor.

The at least one drilling motor may comprise a stator and a rotorrotatably mounted in the stator, the stator being provided with a rodrecess and an exhaust port, the rotor being provided with a rotorchannel and at least one channel for conducting motive fluid from therotor channel to a chamber between the rotor and the stator, the rodrecess being provided with a rod which, in use, forms a seal between thestator and the rotor.

Although not essential it is highly desirable that the rotor be providedwith a seal for engagement with the stator.

Preferably, the seal is made from a material selected from the groupconsisting of plastics materials, polyethylethylketone, metal, copperalloys and stainless steel.

Advantageously, the rod is made from a material selected from the groupconsisting of plastics materials, polyethylethylketone, metal, copperalloys and stainless steel.

Preferably, the stator is provided with two rod recesses which aredisposed opposite one another, and two exhaust ports which are disposedopposite one another, each of the rod recesses being provided with arespective rod, the rotor having two seals which are disposed oppositeone another.

The drilling motor may advantageously comprise two drilling motorsarranged with their respective rotors connected together each motorcomprising a stator and a rotor rotatably mounted in the stator, thestator being provided with a rod recess and an exhaust port, the rotorbeing provided with a rotor channel and at least one channel forconducting motive fluid from the rotor channel to a chamber between therotor and the stator, the rod recess being provided with a rod which, inuse, forms a seal between the stator and the rotor.

Preferably, the drilling motors are connected in parallel, although theycould be connected in series if desired.

Advantageously, the drilling motors are arranged so that, in use, onedrilling motor operates out of phase with the other. Thus, in apreferred embodiment each drilling motor has two chambers and thechambers in the first drilling motor are 90° out of phase with thechambers in the second drilling motor. Similarly, in an embodiment inwhich each drilling motor has four chambers, the chambers in the firstdrilling motor would preferably be 45° out of phase with the chambers onthe second drilling motor. This arrangement helps ensure a smooth poweroutput and inhibits stalling.

Alternatively, the at least one drilling motor may be a “Moineau”,hydraulic or a suitably adapted electric motor.

The impellers may be driven by means of a gearbox or by exploitation ofthe opposing reactive torque on a drive body of the motor.

When the reactive torque upon the motor body is utilised, at least oneimpeller may be connected to an output shaft of said motor, while atleast one other impeller may be connected to the motor body.

Alternatively the impellers may be driven by a pair of motors operatingin opposite directions. In such case said motors and impellers arebalanced and equal.

The underwater excavation apparatus may further comprise an agitatordevice having mechanical disturbance means and fluid flow disturbancemeans.

The underwater excavation apparatus may, in use, be suspended from asurface vessel or mounted upon a sled of the type currently known foruse in subsea excavation operations.

According to a second aspect of the present invention there is providedan underwater apparatus comprising a hollow body having a pair of inletscommunicating with an outlet, at least one pair of impellers rotatablymounted in the hollow body and means for driving the impellers, theinlets being substantially symmetrically disposed around an axisextending from the outlet, wherein the inlets are not horizontallyopposed to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows a cross-sectional side view of a first embodiment of anexcavation apparatus according to the present invention;

FIG. 2 shows a longitudinal cross-sectional view of one embodiment of adrilling apparatus for use in the excavation apparatus in FIG. 1according to the present invention;

FIGS. 3A-3D are cross-sectional views along line A—A of FIG. 2 showing arotor of the motor in four different positions; and

FIGS. 4A-4D are cross-sectional views along line B—B of FIG. 2 showingthe rotor in four different positions.

FIG. 5 shows a cross-sectional side view of a second embodiment of anexcavation apparatus according to the present invention;

FIG. 6 shows a cross-sectional side view of a third embodiment of anexcavation apparatus according to the present invention.

DETAILED DESCRIPTION OF INVENTION

Referring to FIG. 1, there is shown a first embodiment of an underwaterexcavation apparatus 300 a according to the present invention. Theapparatus 300 a comprises a hollow body 370 a formed from a pair ofhorizontally opposed inlet ducts 371 a and an outlet duct 373 a, a drivemotor 310 a and a pair of impellers 335 a, 340 a.

The apparatus 300 a is further provided with deflection baffles 302 awithin the hollow body 370 a, suspension brackets 306 a to enable theapparatus 300 a to be suspended from a surface vessel, guide vanes 386 ato regulate the flow of fluid past the impellers 335 a, 340 a, andsafety grids 385 a to seek to prevent the ingress of solid matter whichmay damage the impellers 335 a, 340 a.

In this first embodiment, the drive motor 310 a is provided along anaxis common to the horizontally opposed inlet ducts 371 a and impellers335 a, 340 a. An output shaft 330 a of the motor 310 a is connected to afirst impeller 335 a while the second impeller 340 a is attached to ashaft 342 a connected via a swivel 325 a to an outer housing of thedrive motor 310 a.

In use, motive fluid is supplied to the motor 310 a via fluid inlet 308a which in turn causes the output shaft 330 a and impeller 335 a torotate. Reactive torque from this rotation causes the outer housing ofthe drive motor 310 a to rotate in a direction opposite to that of theoutput shaft 330 a. This in turn results in the rotation of the secondimpeller 340 a. The impellers 335 a, 340 a are configured such that,despite rotating in opposite directions, they each provide an equal flowrate of water into the hollow body 370 a. Water drawn into the hollowbody 370 a thus is directed via the deflection baffles 302 a through theoutlet duct 373 a and towards the seabed 400 a.

The shaft 342 a and swivel 325 a may, in an alternative embodiment, bereplaced by a second motor which directly drives the impeller 340 a, ashereinbefore described with reference to FIG. 5.

The excavation device 300 a may be suspended, for example, from the bowor stern of a surface vessel, or through a moonpool of a dedicatedsubsea operations vessel.

In an alternative embodiment the device 300 a may be provided upon asled (not shown) of the type currently used for subsea operations. Theexcavation apparatus 300 a may further be provided with an agitatordevice (not shown) having mechanical disturbance means and fluid flowdisturbance means.

In an advantageous embodiment the motor 310 comprises a drilling motor,such as that disclosed in WO95/19488, the content of which isincorporated herein by reference.

The drilling motor 310 may comprise a first motor 20 and a second motor50.

The first motor 20 comprises a stator 21 and a rotor 23. A top portion22 of the rotor 23 extends through an upper bearing assembly 24 whichcomprises a thrust bearing 26 and seals 25.

Motive fluid, e.g. water, drilling mud or gas under pressure, flows downthrough a central sub channel 12 into a central rotor channel 27, andthen out through rotor flow channels 28 into action chambers 31 and 32.

Following a motor power stroke, the motive fluid flows through exhaustports 33 in stator 21, and then downwardly through an annular channelcircumjacent the stator 21 and flow channels 35 in a lower bearingassembly 34. A portion 36 of the rotor 23 extends through the lowerbearing assembly 34 which comprises a thrust bearing 37 and seals 38.

The ends of the stator 21 are castellated and the castellations engagein recesses in the respective upper bearing assembly 24 and lowerbearing assembly 34 respectively to inhibit rotation of the stator 21.The upper bearing assembly 24 and lower bearing assembly 34 are a tightfit in an outer tubular member 14 and are held against rotation bycompression between threaded sleeves 16 and 84.

A splined union 39 joins a splined end of the rotor 23 to a splined endof a rotor 53 of the second motor 50. The second motor 50 has a stator51.

A top portion 52 of the rotor 53 extends through an upper bearingassembly 54. Seals 55 are disposed between the upper bearing assembly 54and the exterior of the top portion 52 of the rotor 53. The rotor 53moves on thrust bearings 56 with respect to the upper bearing assembly54.

Motive fluid flows into a central rotor channel 57 from the centralrotor channel 27 and then out through rotor flow channels 58 into actionchambers 61 and 62. Following a motor power stroke, the motive fluidflows through exhaust ports 63 in stator 51, and then downwardly throughan annular channel circumjacent the stator 51 and flow channels 65 in alower bearing assembly 64. A portion 66 of the rotor 53 extends througha lower bearing assembly 64. The rotor 53 moves on thrust bearings 67with respect to the lower bearing assembly 64 and seals 68 seal therotor-bearing assembly interface. Also motive fluid which flowed throughthe flow channels 35 in the lower bearing assembly 34, flows downwardlythrough channels 79 in the upper bearing assembly 54, past stator 51 andthrough flow channels 65 in the lower bearing assembly 64.

The upper bearing assembly 54 and lower bearing assembly 64 are a tightfit in an outer tubular member 18 and are held against rotation bycompression between threaded sleeve 84 and a lower threaded sleeve (notshown).

FIGS. 2A-2D and 3A-3D depict a typical cycle for the first and secondmotors 20 and 50 respectively, and show the status of the two motorswith respect to each other at various times in the cycle. For example,FIG. 2C shows an exhaust period for the first motor 20 while FIG. 3C, atthat same moment, shows a power period for the second motor 50.

As shown in FIG. 2A, motive fluid flowing through the rotor flowchannels 28 enters the action chambers 31 and 32. Due to the geometry ofthe chambers (as discussed below) and the resultant forces, the motivefluid moves the rotor in a clockwise direction as seen in FIG. 2B. Theaction chamber 31 is sealed at one end by a rolling vane rod 71 whichabuts an exterior surface 72 of the rotor 23 and a portion 74 of a rodrecess 75.

At the other end of the action chamber 31, a seal 76 on a lobe 77 of therotor 23 sealingly abuts an interior surface of the stator 21.

As shown in FIG. 2B, the rotor 23 has moved to a point near the end of apower period.

As shown in FIG. 2C, motive fluid starts exhausting at this point in themotor cycle through the exhaust ports 33.

As shown in FIG. 2D, the rolling vane rods 71 and seals 76 have sealedoff the action chambers and motive fluids flowing thereinto will rotatethe rotor 23 until the seals 76 again move past the exhaust ports 33.

The second motor 50 operates as does the first motor 20; but, aspreferred, and as shown in FIGS. 3A-3D, the two motors are out of phaseby 90° so that as one motor is exhausting motive fluid the other isproviding power.

The seals 76 are, in one embodiment, made of polyethylethylketone(PEEK). The rolling vane rods 71 are also made from PEEK. The rotors(23, 25) and stators (21, 51) are preferably made from corrosionresistant materials such as stainless steel.

When a seal 76 in the first motor 20 rotates past an exhaust port 33,the motive fluid that caused the turning exits and flows downward, thenthrough the channels 79, past the exhaust ports 63 and the flow channels65.

It should be appreciated that although in the disclosed embodiment thedrilling motor 310 comprises two motors 20, 50, with suitableadaptation, the drilling motor 310 may comprise only one motor 20 or 50.

Referring now to FIG. 5, there is shown a second embodiment of anunderwater excavation apparatus 300 b according to the presentinvention. Like parts of the apparatus 300 a are identified by numeralsused to identify parts of the apparatus 300 a of FIG. 1, exceptsubscripted with “b” rather than “a”.

The apparatus 300 b differs from the apparatus 300 a in that the shaft342 a and swivel 325 a are replaced by a second motor 310′b and aT-coupling 326 b. Thus in this embodiment the impellers 335 b, 340 b aredriven by respective motors 310 b, 310′b. In use, motive fluid issupplied to motors 310 b, 310′b via fluid inlet 308 b and T-coupling 326b.

Referring now to FIG. 6, there is shown a second embodiment of anunderwater excavation apparatus 300 c according to the presentinvention. Like parts of the apparatus 300 b are identified by numeralsused to identify parts of the apparatus 300 b of FIG. 5, exceptsubscripted with “c” rather than “b”.

The apparatus 300 c differs from the apparatus 300 b in that whereas inapparatus 300 b the inlets 371 b are horizontally opposed, in apparatus300 c the inlets are substantially symmetrically disposed around an axisextending from outlet 373 c, such that the apparatus 300 c issubstantially “Y” shaped. In this embodiment there is, therefore,provided a Y-coupling 326 c.

The embodiments of the invention hereinbefore described are given by wayof example only, and are not meant to limit the scope of the inventionin any way. It should be particularly appreciated that the drillingmotor 310 is suitable for use in any of the disclosed embodiments.

What is claimed is:
 1. An underwater excavation apparatus comprising ahollow body having at least one pair of inlets and at least one outlet,at least one pair of impellers rotatably mounted in the hollow body, andmeans for driving the impellers, wherein the pair of inlets aresubstantially symmetrically disposed around an axis extending from theat least one outlet, and wherein the driving means cause the impellersto be driven in contrary rotating directions.
 2. An underwaterexcavation apparatus as claimed in claim 1, wherein there is providedone pair of inlets.
 3. An underwater excavation apparatus as claim inclaim 1, wherein the apparatus comprises a pair of horizontally opposedinlets communicating with a single outlet, the outlet being disposedvertically downwards substantially midway between the two inlets, inuse, such that the excavation apparatus is substantially “T” shaped inprofile.
 4. An underwater excavation apparatus as claimed in claim 1,wherein the apparatus comprises a pair of inlets communicating with asingle outlet, the inlets being substantially symmetrically disposedaround an axis extending from the outlet, the outlet being disposedvertically downwards substantially midway between the two inlets, inuse, such that the excavation apparatus is substantially “Y” shaped inprofile.
 5. An underwater excavation apparatus as claimed in claim 2,wherein the apparatus comprises a pair of horizontally opposed inletscommunicating with a single outlet, the outlet being disposed verticallydownwards substantially midway between the two inlets, in use, such thatthe excavation apparatus is substantially “T” shaped in profile.
 6. Anunderwater excavation apparatus as claimed in claim 2, wherein the meansfor driving the impellers includes at least one drilling motor.
 7. Anunderwater excavation apparatus as claimed in claim 1, wherein one ofthe impellers is provided within one of the inlets and another of theimpellers is provided within another of the inlets.
 8. An underwaterexcavation apparatus comprising a hollow body having at least one pairof inlets and at least one outlet, at least one pair of impellersrotatably mounted in the hollow body, and means for driving theimpellers, wherein the pair of inlets are substantially symmetricallydisposed around an axis extending from the at least one outlet, andwherein the means for driving the impellers includes at least onedrilling motor.
 9. An underwater excavation apparatus as claimed inclaim 8, wherein the at least one drilling motor comprises a stator anda rotor rotatably mounted in the stator, the stator being provided witha rod recess and an exhaust port, the rotor being provided with a rotorchannel and at least one channel, for conducting motive fluid from therotor channel to a chamber between the rotor and the stator, the rodrecess being provided with a rod which, in use, forms a seal between thestator and the rotor.
 10. An underwater excavation apparatus as claimedin claim 9, wherein the rotor is provided with a seal for engagementwith the stator.
 11. An underwater excavation apparatus as claim inclaim 10, wherein the seal is made from a material selected from thegroup consisting of plastics materials, polyethylethylketone, metal,copper alloys and stainless steel.
 12. An underwater excavationapparatus as claimed in claim 9, wherein the rod is made from a materialselected from the group consisting of plastics materials,polyethylethylketone, metal copper alloys and stainless steel.
 13. Anunderwater excavation apparatus as claimed in claim 9, wherein thestator is provided with two rod recesses which are disposed opposite oneanother, and two exhaust ports which are disposed opposite one another,each of the rod recesses being provided with a respective rod, the rotorhaving two seals which are disposed opposite one another.
 14. Anunderwater excavation apparatus as claimed in claim 10, wherein the rodis made from a material selected from the group consisting of plasticsmaterials, polyethylethylketone, metal, copper alloys and stainlesssteel.
 15. An underwater excavation apparatus as claimed in claim 10,wherein the stator is provided with two rod recesses which are disposedopposite one another, and two exhaust ports which are disposed oppositeone another, each of the rod recesses being provided with a respectiverod, the rotor having two seals which are disposed opposite one another.16. An underwater excavation apparatus as claimed in claim 11, whereinthe rod is made from a material selected from the group consisting ofplastics materials, polyethylethylketone, metal, copper alloys andstainless steel.
 17. An underwater excavation apparatus as claimed inclaim 11, wherein the stator is provided with two rod recesses which aredisposed opposite one another, and two exhaust ports which are disposedopposite one another, each of the rod recesses being provided with arespective rod, the rotor having two seals which are disposed oppositeone another.
 18. An underwater excavation apparatus as claimed in claim12, wherein the stator is provided with two rod recesses which aredisposed opposite one another, and two exhaust ports which are disposedopposite one another, each of the rod recesses being provided with arespective rod, the rotor having two seals which are disposed oppositeone another.
 19. An underwater excavation apparatus as claimed in claim8, wherein the at least one drilling motor comprises two drilling motorsarranged with their respective rotors connected together each motorcomprising a stator and a rotor rotatably mounted in the stator, thestator being provided with a rod recess and an exhaust port, the rotorbeing provided with a rotor channel and at least one channel forconducting motive fluid from the rotor channel to a chamber between therotor and the stator, the rod recess being provided with a rod which, inuse, forms a seal between the stator and the rotor.
 20. An underwaterexcavation apparatus as claimed in claim 19, wherein the drilling motorsare connected in parallel or in series.
 21. An underwater excavationapparatus as claimed in claim 19, wherein the drilling motors arearranged so that, in use, one drilling motor operates out of phase withthe other.
 22. An underwater excavation apparatus as claimed in claim20, wherein the drilling motors are arranged so that, in use, onedrilling motor operates out of phase with the other.
 23. An underwaterexcavation apparatus comprising a hollow body having at least one pairof inlets and at least one outlet, at least one pair of impellersrotatably mounted in the hollow body, and means for driving theimpellers, wherein the pair of inlets are substantially symmetricallydisposed around an axis extending from the at least one outlet, andwherein the apparatus comprises a pair of horizontally opposed inletscommunicating with a single outlet, the outlet being disposed verticallydownwards substantially midway between the two inlets, in use, such thatthe excavation apparatus is substantially “T” shaped in profile.
 24. Anunderwater excavation apparatus as claimed in claim 23, wherein themeans for driving the impellers includes at least one drilling motor.25. An underwater excavation apparatus as claimed in claim 23, whereinat least one impeller is provided within each outlet.
 26. An underwaterexcavation apparatus comprising a hollow body having at least one pairof inlets and at least one outlet, at least one pair of impellersrotatably mounted in the hollow body, and means for driving theimpellers, wherein the pair of inlets are substantially symmetricallydisposed around an axis extending from the at least one outlet, andwherein one of the impellers is provided within one of the inlets andanother of the impellers is provided within another of the inlets. 27.An underwater excavation apparatus as claimed in claim 26, wherein thereis provided one pair of inlets.
 28. An underwater excavation apparatusas claimed in claim 26, wherein the apparatus comprises a pair ofhorizontally opposed inlets communicating with a single outlet, theoutlet being disposed vertically downwards substantially midway betweenthe two inlets, in use, such that the excavation apparatus issubstantially “T” shaped in profile.
 29. An underwater excavationapparatus as claimed in claim 26, wherein the apparatus comprises a pairof inlets communicating with a single outlet, the inlets beingsubstantially symmetrically disposed around an axis extending from theoutlet, the outlet being disposed vertically downwards substantiallymidway between the two inlets, in use, such that the excavationapparatus is substantially “Y” shaped in profile.
 30. An underwaterexcavation apparatus as claimed in claim 26, wherein the means fordriving the impellers includes at least one drilling motor.
 31. Anunderwater excavation apparatus as claimed in claim 26, wherein themeans for driving the impellers includes at least one drilling motor.32. An underwater excavation apparatus comprising a hollow body havingat least one pair of inlets and at least one outlet, at least one pairof impellers rotatably mounted in the hollow body, and means for drivingthe impellers, wherein the pair of inlets are substantiallysymmetrically disposed around an axis extending from the at least oneoutlet, and wherein the apparatus comprises a pair of inletscommunicating with a single outlet, the inlets being substantiallysymmetrically disposed around an axis extending from the outlet, theoutlet being disposed vertically downwards substantially midway betweenthe two inlets, in use, such that the excavation apparatus issubstantially “Y” shaped in profile.
 33. An underwater excavationapparatus as claimed in claim 32, wherein the means for driving theimpellers includes at least one drilling motor.
 34. An underwaterexcavation apparatus as claimed in claim 32, wherein the inlets are eachinclined substantially 45° from the axis extending from the outlet. 35.An underwater excavation apparatus comprising a hollow body having atleast one pair of inlets and at least one outlet, at least one pair ofimpellers rotatably mounted in the hollow body, and means for drivingthe impellers, wherein the pair of inlets are substantiallysymmetrically disposed around an axis extending from the at least oneoutlet, and wherein the impellers are driven by means of a gearbox. 36.An underwater excavation apparatus as claimed in claim 35, wherein thereactive torque upon the said drive body is utilized, at least oneimpeller is connected to an output shaft of said motor, while at leastone other impeller is connected to the said drive body.
 37. Anunderwater excavation apparatus comprising a hollow body having one pairof inlets and at least one outlet, at least one pair of impellersrotatably mounted in the hollow body, and means for driving theimpellers, wherein the pair of inlets are substantially symmetricallydisposed around an axis extending from the at least one outlet, whereinthe means for driving the impellers includes at least one drillingmotor.
 38. An underwater excavation apparatus comprising a hollow bodyhaving at least one pair of inlets and at least one outlet, at least onepair of impellers rotatably mounted in the hollow body, and means fordriving the impellers, wherein the pair of inlets are substantiallysymmetrically disposed around an axis extending from the at least oneoutlet, and wherein the impellers are driven by a pair of motorsoperating in opposite directions.
 39. An underwater excavation apparatuscomprising a hollow body having at least one pair of inlets and at leastone outlet, at least one pair of impellers rotatably mounted in thehollow body, and means for driving the impellers, wherein the pair ofinlets are substantially symmetrically disposed around an axis extendingfrom the at least one outlet, and wherein the underwater excavationapparatus further comprises an agitator device having mechanicaldisturbance means and fluid flow disturbance means.
 40. An underwaterexcavation apparatus comprising a hollow body having at least one pairof inlets and at least one outlet, at least one pair of impellersrotatably mounted in the hollow body, and means for driving theimpellers, wherein the pair of inlets are substantially symmetricallydisposed around an axis extending from the at least one outlet, andwherein in use the underwater excavation apparatus is suspended from asurface vessel or mounted upon a sled.
 41. An underwater excavationapparatus comprising a hollow body having at least one pair of inletsand at least one outlet, at least one pair of impellers rotatablymounted in the hollow body, and means for driving the impellers, whereinthe pair of inlets are substantially symmetrically disposed around anaxis extending from the at least one outlet, and wherein the impellersare driven by means of exploitation of the opposing reactive torque on adrive body of at least one motor.